feat(series): operator-set sparse page numbers + gap blocks (#789 tweak)

Replaces the auto-renumbered 1..N position key with operator-OWNED page
numbers: sparse, gaps allowed, editable, never auto-renumbered. Order follows
the numbers; unnumbered pages sort to the tail. This is the fix for the model
that clobbered hand-set numbers on the flatten — numbers are now data, not a
derived sequence.

- series_service: drop the renumber-on-reorder/remove; order by page_number
  NULLS LAST; new set_page_number(image_id, n|None); list_pages returns `gaps`
  (one entry per missing-number run) + each pending group's parsed `start_page`;
  set_cover renumbers below the current min; place_pending(image_ids, start_page)
  numbers placed pages sequentially from the start (drop junk first → numbers
  line up); add_post stamps the parsed start on staged pages.
- api/tags: POST /series/<id>/pages/number (set one page's number); /pending/
  place takes start_page; removed /reorder.
- frontend: per-card editable number input; one gap block per gap with
  drop-on-edge to assign the adjacent number (middle → type); append drop zone;
  pending tray gets a "from page N" field + "Place from page N".
- tests reworked: sparse numbers + gaps, place-from-start, set-page-number route.

No migration; nothing destructive.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_api_series.py

@@ -44,10 +44,10 @@ async def test_add_over_cap_400(client):
 
 
 @pytest.mark.asyncio
-async def test_reorder_mismatch_400(client):
-    sid = await _series(client, "Re")
+async def test_set_page_number_requires_image_id(client):
+    sid = await _series(client, "Num")
     resp = await client.post(
-        f"/api/series/{sid}/reorder", json={"image_ids": [1, 2, 3]}
+        f"/api/series/{sid}/pages/number", json={"page_number": 5}
     )
     assert resp.status_code == 400